This is not mercedes related, but it is close enough people here may have some input.

The vehicle is a vw golf with an AAZ swapped in, with rather questionable wiring, quite a bit of rust, unknown mileage, etc, that my sister (who has pretty much no car knowledge) purchased at the advice of some friends for quite a bit of money. It has never started easily, and the glow plugs dont appear to work. She has driven it for about a year like this, and she starts it by cranking until it starts to fire, then holding it at high RPM until it will stay running at idle, then drive away. Lately it has gotten harder to start, and since it doesn't have a functional parking brake she also noticed that it rolls much more easily in gear than it did before. It is currently her only car which she uses to get to work, but the same people who advised her to purchase it want to sell the AAZ and put a TDI in it soon and don't think fixing the glow plugs is worth the trouble, so they told her to just use starting fluid to start it. I have my own opinions on the whole thing, but I haven't had anything to do with it at all.